Process for producing alumina monohydrate from basic aluminum nitrates

ABSTRACT

Process for producing alumina monohydrate and aluminum nitrate from basic aluminum nitrates by preparing an aqueous solution of the basic aluminum nitrate having a nitrate concentration low enough to prevent solution of Al2O3, heating the solution under autogenous pressure to convert the free alumina in the basic aluminum nitrate to solid alumina monohydrate, and recovering the solid alumina monohydrate.

I United States Patent 1191 1111 Bruen et al. 1 1 Dec. 9, 1975 1 PROCESSFOR PRODUCING ALUMINA [56] References Cited MONOHYDRATE FROM BASICALUMINUM UNITED STATES PATENTS NITRATES 1,792,410 2/1931 Buchner 423/12375 Inventors; Chm-ks p Bruen Basking Ridge; 2,196,016 4/1940 Huehn eta1. 423/495 Donald [L Kelly Gladstone both of 2,915,475 12/1959 Bugosh 11 1 .4 252/313 NJ 3,056,747 10/1962 Arthur, 11.... 264/87 I 3,366,4461/1968 Kelly et a1 4 .1 423/390 [73] Assignee: Reynolds Metals Company,3,387,921 6/1968 Amano et al. H 11/400 Richmond, Va. Filed: o 6, 1974Primary Exam1nerHerbert T. Carter [21] Appl. No.: 521,381 [57] ABSTRACTRelated Us. Application Data Process for producing alumina monohydrateand aluminum nitrate from basic aluminum nitrates by pre- [62] 3223 1489w June 1970' paring an aqueous solution of the basic aluminum nitratehaving a nitrate concentration low enough to [52] us CL 423/390,423/395, 423/400, prevent solution of A1 0 heating the solution under I1 i i v 423/63 autogenous pressure to convert the free alumina in the[51] Int Cl 2 C018 2l/42 C0": 7/30 basic aluminum nitrate to solidalumina monohydrate, 58 Field 01 eta 11 111511111111111. 423/631 360 395400 and mowing Solid alumina mOmhYdrate- 1 Claim, No Drawings PROCESSFOR PRODUCING ALUMINA MONOIIYDRATE FROM BASIC ALUMINUM NITRATES This isa division of application Ser. No. 48,919 filed June 18, 1970, now U.S.Pat. No. 3,864,462.

This invention relates to a process for the production of aluminamonohydrate from basic aluminum nitrates.

It is known to produce anhydrous alumina by a multistage process whichinvolves the nitric acid digestion of aluminous ores followed bycrystallization of the resulting aluminum nitrate as the nonahydrateAI(NO .9- H 0, and decomposition and denitrification of the aluminumnitrate nonahydrate to alumina and nitric acid by the application ofheat.

Denitrification of aluminum nitrate nonahydrate has been carried out inthe past by direct application of heat at atmospheric pressure toaluminum nitrate nonahydrate crystals, which melt at 735C. and boil atl35C. with decomposition. [n such atmospheric denitrification processes,decomposition can be continued with agitation of the melt until about50% of the N in the melt has been removed, together with water vaporfrom the crystals, and recovered as nitric acid (HN0 of about 50%concentration. The remaining melt has a boiling point of about 148C. andsuch a high viscosity (ca 10,000 centipoises or higher) that agitationbecomes difficult or impracticable, resulting in clinging of the melt tothe walls of the container, and local overheating of the melt whichfurther complicates denitrification. Moreover, when the melt is cooled,it is a sticky, gummy, mass with a softening point of about 123C. whichresists further denitrification, since several hours heating at l C. arerequired to remove as little as 1% of the remaining nitric acid values.

In U.S. Pat. No. 3,366,446 of Donald H. Kelly and Anthony W. Yodis,granted Jan. 30, 1968, there is described a process for denitrificationof aluminum nitrate nonahydrate which avoids the formation of a gummymass and permits more complete denitrification than does the priorprocess. The Kelly et al. process involves the decomposition of aluminumnitrate nonahydrate by melting the aluminum nitrate nonahydrate, heatingthe molten aluminum nitrate nonahydrate to a temperature above itsdecomposition temperature but not above 230C., under superatmosphericpressure of at least about 25 psig and sufficient to maintain saiddecomposition temperature while removing gaseous nitric aciddecomposition products until between about 45 and about 55% of the l-lNOof the aluminum nitrate nonahydrate has been removed, followed byflashing i.e. reducing the pressure quickly to atmospheric, to produce amolten residue which, on cooling, becomes a friable solid, cooling themolten residue to a temperature below its softening point, holding thesolid melt below the softening temperature of the melt for one-half houror more, thus producing a friable non-melting solid, and further heatingthe nonmelting solid to decompose it into N 0,, and solid M 0 Thedenitrification process of the above Kelly et al. patent thus providesmarked advantages over the prior art, especially in the production offriable, non-melting solids which are readily handled, instead of thegummy products ofthe prior art, and permits greater nitric acidrecovery. It was found, however, that, in the Kelly et al. process, atthe temperatures required to break down the non-melting solid into thealumina and nitric acid values, substantial proportions of undesiredgaseous oxides of nitrogen, NO and N0 are formed along with the desirednormally liquid N 0,. The gaseous nitrogen oxides are troublesome sincethey do not dissolve in water; they do not form nitric acid directly asdoes N 0 and they must be oxidized in a separate converter in order torecover them as nitric acid.

It is an object of the present invention to provide a process for theproduction of alumina monohydrate in which virtually no lower oxides ofnitrogen are produced.

Another object of the invention is to provide an alumina recoveryprocess wherein A1 0 values are produced in non-dusting readilyrecoverable particle size.

A still further object is to provide a process for producing aluminafrom basic aluminum nitrates, wherein virtually no troublesome gaseousoxides of nitrogen are produced.

These and other objects are accomplished according to our inventionwherein basic aluminum nitrates such as those prepared by the partialdenitrification of aluminum nitrate nonahydrate to a point where thebasic aluminum nitrates have a combined nitrate content between about 25and about of that combined in normal aluminum nitrate Al(NO aresubjected to hydrothermal decomposition to alumina monohydrate AI,O .H,Oand aluminum nitrate by heating the basic aluminum nitrates in thepresence of an excess of water sufficient to provide a concentration ofcombined nitrates, in solution, too low to dissolve substantialproportions of free A1 0 preferably not more than about 25% nitrates byweight (calculated as N0 to a temperature of at least about C. underautogenous pressure for a period sufficient to convert substantially allof the free A1 0 in the basic aluminum nitrate to solid aluminamonohydrate, and recovering the solid alumina monohydrate thus produced.By free alumina is meant that portion of the aluminum not bound by thenitrate radical and calculated as A1 0 1n carrying out the processaccording to our invention, aqueous aluminum nitrate, existing either asthe nonahydrate Al(NO,-,),.9H,O or greater aqueous dilutions thereof, ispartially denitrified to remove between about 25% and about 75% of thenitric acid values, by any suitable means, for example by heating toboiling under atmospheric pressure, or, preferably, by digestion underpressure as described in U.S. Pat. No. 3,366,446.

The partially denitrified product is a mixture of basic aluminumnitrates containing substantial proportions of a dibasic aluminumnitrate of the formula la. Al(Ol-l),NO

which may also be viewed as lb. Al,O,.Al(NO,),,.3l-l,0 We haveidentified compound la by X-ray diffraction as identical with thatdescribed in Table X1, page 32, of IDO 14,574 Quarterly report for 3rdquarter 1961 of the Idaho operations office, U.S. Atomic EnergyCommission. The partially denitrified product also contains one or moreother basic aluminum nitrates of unidentitied composition, possiblyincluding the monobasic aluminum nitrate lla. Al(OH)(NO which may alsobe viewed as llb. Al,O .4Al(NO .3l- ,O Proportions of the differentaluminum nitrates vary depending on degree of denitrification. We carrythis denitrification to between about 75 and about 25% of completion,thus producing basic aluminum nitrates containing between about 25 andabout 75% the proportion of bound nitrates contained in normal aluminumnitrate Al(NO Thus the basic aluminum nitrate starting materials of ourinvention can be viewed as having the empirical formula shown belowexpressed in terms of its free" alumina content as llla. AI,O,.xAI(NO.yH,O wherein x is a numeral from 0.667 to 6.0 inclusive, y is a numeralfrom 4 to 38 inclusive.

Expressed in terms of alumina, N and water this formula becomes:

lllb. Al,O,.pN,O .zH O wherein p is a numeral ranging from 0.75 to 2.25inclusive, z is a numeral ranging from 3.0 to 9.5 inclusive.

At 50% denitrification the empirical formula of the basic nitrates canbe written as Formula lb shows that the dibasic aluminum nitratecontains one mole of free alumina, A1 0 for every mole of boundaluminum, AI(NO:,),; formula [lb shows that the hypothetical monobasicaluminum nitrate contains one mole of free alumina for every 4 moles ofbound aluminum. The mixtures of basic aluminum nitrates as shown inempirical formula llla have between about 0.667 and 6.0 moles of boundaluminum per mole of free alumina.

The degree of denitrification of the basic aluminum nitrate startingmaterial of our invention may vary, but preferably should have at leastabout 25% of the N 0 of the starting Al(NO,-.,) removed, i.e. the massshould contain no more than about 75% the amount of bound nitratescompared to the amount in normal aluminum nitrate Al(NO preferablybetween about 75 and about 25%. Degrees of denitrification lower thanabout 25% are unsatisfactory as the alumina remains in solution in thealuminum nitrate. Aluminum nitrate charges which have been denitrifiedto the extent of about 25% and higher produce a solid precipitate andwhen heated according to our invention give excellent yields of alumina.Those charges which have been denitrified to at least about 40 yieldabout 100% the recoverable alumina. 0n the other hand, we find ituneconomical to carry the denitrification beyond about 75% denitrified,since at denitrifications above this value, substantial proportions ofthe nitrate values are decomposed into unwanted gaseous lower oxides ofnitrogen, NO and M0,. We therefore prefer to use denitrified chargesfrom which between about 25 and about 75% of the MO, has been recovered,i.e. which contain chemically bound nitrates in proportions betweenabout 75 and about 25% of the nitrates in normal aluminum nitrate,Al(NO,),.

ln practicing our invention, the partially denitrified product describedabove, is diluted with a quantity of water sufficient to produce amedium having an aluminum nitrate concentration too low to dissolveappreciable quantities of Al,0,. Thus we provide a medium containing notmore than about 25% by weight of combined nitrates calculated as NO,,based on the total weight of the solution, remaining in the denitrifiedliquor, preferably between about 20% and about combined nitrates. Sincewe have found that both the basic aluminum nitrates and free Al,0; aresoluble in aqueous aluminum nitrate solutions of concentrations aboveabout 25% of combined nitrates (calculated as N0 dilutions to below thisconcentration are essen- TABLE I Moles of 11,0 Per Mole of N0, Requiredto be Added Basic Aluminum For 25% HNO, For 10% HNO, Nitrate ProductSolution Solution 45% denitrified 7.2 28.2

50% denitrified 7.7 28.3

60% denitrified 7.15 28.1

For intermediate NO, concentrations between 25 and 10%, intermediateamounts of water will be required. It is apparent from the above Table lthat the molar ratios of water to combined N0 present, required toobtain a predetermined dilution is virtually constant regardless of theextent of denitrification. Thus as clearly indicated in the table,suitable dilutions will range between about 7.2 moles and about 28.2moles of water per mole of bound NO, 14.4 moles to 56.4 moles per moleof N 0,).

The diluted basic aluminum nitrate is then heated under autogenouspressure to at least about C., for a sufficient time to break down thebasic aluminum nitrate substantially completely and to causeprecipitation of virtually all the free alumina as alumina monohydrateAl,0,.l-l,0, virtually devoid of any nitrate.

Temperatures during the above hydrothermal digestion step are criticaland should be at least high enough to effect substantially completedissociation or disproportionation of the basic aluminum nitrate of thestarting product to alumina monohydrate and aluminum nitrate, to avoidinclusion in the precipitate of any nitrogen radicals which would bereleased as lower nitrogen oxides in the eventual calcination of thealumina monohydrate to alumina. Temperatures of at least about 180C., asbetween about 180C. and about 235C, are satisfactory; temperaturesbetween about C. and about 210C. are preferred. Temperatures lower thanabout 180C. tend to promote the precipitation of the basic nitrates.Temperatures of 180C. or higher tend to inhibit basic nitrateprecipitation. Time of heating will depend somewhat on the temperatureemployed; lower temperatures requiring somewhat longer heating timesthan higher temperatures. Usually heating periods of at least aboutone-half hour are required, and heating periods between about 1.5 hoursand about 5 hours at the preferred temperatures are usually sufficientto produce substantially complete dissociation of the basic aluminumnitrates. Upon heating at 160C. for a period of about 3 hours aprecipitate is obtained which may contain as much as 25% or more byweight of nitrates calculated as M0,. Thus this temperature is too lowfor successful operation. Upon heating at 190C. for the same period theresidual nitrate is only about 10% whereas at 200C. only about 2.5% ofnitrates appear. At about 210C. the residual nitrates may be as low asabout 1.5%. These low amounts of nitrate are volatilized on finalcalcining as NO and N0,.

Proportions of residual nitrates of 10% or below, present no operatingdifficulties and represent a nitric acid loss of only small percentages,i.e., no more than about 3% of the total nitric acid values in the basicaluminum nitrates or about 1.5% of the total l-[NO values in the normalaluminum nitrate. Temperatures higher than 210C., for example up toabout 235C. or higher while operationally satisfactory, are unnecessaryfor adequate suppression of nitrate precipitation, and present problemsattendant on the higher pressures produced at these temperatures.

Time of heating the diluted solution is not unduly critical but shouldbe sufficient to insure complete breakdown of the basic aluminumnitrates; usually a period between about one-half hour and about 5 hourswill be sufficient.

The mechanism of the breakdown or disproportionation of the basicaluminum nitrate to alumina monohydrate is not entirely understood butis believed to 1 proceed as shown in one or both of the equationsset Theprecipitated alumina monohydrate is recovered as by filtration or othersuitable means, and may then be calcined to anhydrous alumina by heatingat temperatures of about 350C. and above.

The undenitrified aluminum nitrate remains dissolved in the motherliquor and is suitably recycled to a succeeding aluminum nitratedenitrification step. Thus in commercial operation a steady state isestablished wherein recovery of both alumina and nitric acid values arehigh. Impurities which build up in the aluminum nitrate mother liquor donot appear to contaminate the precipitated alumina monohydrate untiltheir concentrations have reached surprisingly high proportions. At thisstage in a continuous operation sufficient quantities of mother liquorcan be bled off to insure maintenance of impurity composition below alevel which would be harmful to the precipitated alumina monohydrate.

Our discovery that the breakdown of basic aluminum nitrates can be madeto occur, and the provision of conditions under which the aluminamonohydrate will precipitate, leaving virtually all the normal nitratesin solution, is new and surprising and is based on the discovery of thecritical conditions, especially those of temperature and degree ofdilution which provide this surprising freedom from contaminatingnitrates.

The alumina monohydrate precipitated in our hydrothermal process isgenerally a very pure crystalline Al- ,O,.l-l,(), sometimes known asBoehmite, and exhibits the characteristic X-ray diffraction pattern forthis substance.

The precipitated alumina monohydrate crystals formed by directprecipitation in the manner described above, are often extremely small,ranging from a few microns in diameter to perhaps about microns. Whileit is possible to effect a good recovery of these fine crystals, we findit advantageous to provide crystals of somewhat larger particle size forease in handling and prevention of dusting on eventual calcining.

We have found that crystalline A1,0,,.H,O of larger particle sizes canbe produced if seed crystals of the alumina monohydrate are added to thepartially denitrified, basic aluminum nitrate mass before thehydrothermal digestion. An amount of monohydrate seed crystalsequivalent to at least about 35% by weight of free alumina (Al,0,) ispreferred to insure appreciable growth of crystals to make them easilyfilterable and to avoid dusting in the subsequent calcination step,preferably an amount of seed crystals between about 40% and about byweight of the free A1 0, in the basic aluminum nitrate mass to bedigested. ln this manner alumina monohydrate crystals of averageparticle size of at least about 15 microns are obtained.

The following specific examples further illustrate our invention. Partsare by weight except as otherwise noted.

EXAMPLE 1 A charge of 2,500 parts of aluminum nitrate nonahydratecrystals Al(NO,) .9l-l,O, was placed into an autoclave equipped with anagitator, vapor outlet and injection tube. The autoclave was closed andthe charge was heated to 180C, thus developing an internal pressure of50 psig, and causing decomposition of the aluminum nitrate to takeplace. The vapor outlet was opened, and vaporous decomposition products,N 0 and water, were vented as aqueous nitric acid, while agitating themass and maintaining the temperature and pressure at the indicated leveluntil nitric acid equivalent to 52.6% of the amount theoreticallyobtainable from the aluminum nitrate in the charge had been removed andcollected as 52.3% aqueous nitric acid.

The mass at this stage was still fluid, having a viscosity of 900centipoises. The vapor outlet was then closed and 2,250 parts of waterwere introduced into the autoclave, thus providing in the charge a totalaluminum concentration of 9.8% (calculated as A1 0 and a combined BNO,concentration of 17.1% based on the total weight of solution.

After dilution of the charge, the autoclave contents were heated at225C. for 1 hour under autogenous pressure of 300 psig, during whichtime a precipitate formed and accumulated. The resulting slurry wasremoved from the autoclave, filtered under pressure, and the filter cakewas washed with 10,000 parts of water. The filtrate, amounting to 12,650parts, contained 162 parts of aluminum nitrate and 29 parts of A1 0,,the latter equivalent to 16.2% of the free (i.e. denitrified) alumina inthe slurry, indicating that 83.8% of the denitrilied alumina produced inthe initial denitrification step had precipitated. The washed filtercake was dried at C. The dried cake amounted to 193.5 parts andcontained 2.74% residual nitrate as HNO It gave an X-ray pattern foralumina monohydrate, A1,0,.H,O, (Boehmite). The filtrate was recycled toa subsequent batch of diluted autoclave liquor for further work-up.

The recovery of A1 0 was 44.0% of the total aluminum (calculated as A1 0present in the originally charged A1(NO,) .9H,O denitrified. Therecovery of HNO based on the total AI(NO .9H,O was 52.6%. The totalamount of N 0, accounted for as aluminum nitrate in the filtrate and asHNO, in the recovered nitric acid and monohydrate crystals amounted to99.95% of that originally present in the aluminum nitrate chargedindicating only 0.05% of the total N 0 was lost as NO and N0 EXAMPLE 2 Acharge of 1250 parts of Al(NO,) .9H,O was denitrified in the mannerdescribed in Example 1 at 180C. and 50 psig until 50.0% of thetheoretically possible N,O had been removed and collected as 51.6% HNO,

The partially denitrified liquor was diluted in the autoclave with 2630parts of water, thus providing a total aluminum concentration of 2.7%(calculated as A1 and a combined HNO concentration of 10.0%.

After dilution of the charge the autoclave contents were heated at 190C.for 4 hours under autogenous pressure of 155 psig. The autoclave wasdepressurized, the resulting slurry removed and filtered under pressure,yielding a filtrate containing 355 parts aluminum nitrate and 3 parts ofA1 0 the latter equivalent to 3.5% of the free (i.e. denitrified)alumina in the slurry indicating that 96.5% of the denitrified aluminaproduced in the initial denitrification step had precipitated. Thefilter cake amounted to 104 parts and contained 3.0% residual nitrate asHNO It gave the characteristic X-ray pattern for alumina monohydrate.Recovery of A1 0, was 48.25% of the total aluminum present in theoriginally charged AI(NO,),.9H,O and 96.5% of the amount theoreticallyrecoverable from the portion of Al(NO .9H,O denitrified. The recovcry ofHNO based on the total Al(NO .9H,O was 50.0%.

EXAMPLE 3 1n the same manner as that described in Example 1 above, 2,500parts of Al(NO .9H,O was denitrified at 180C. and 50 psig until 60.0% ofthe theoretically possible N 0, had been removed and collected as 57.6%HNO The partially denitrified liquor was diluted in the autoclave with1550 parts of water thus providing a total aluminum concentration of5.4% (calculated as AI,O,) and a combined HNO concentration of 20.0%.

The diluted charge was then heated to 235C. under autogenous pressure of350 psig for 3/4 hour. The resulting slurry was removed from theautoclave and fil tered, yielding 2,280 parts of filtrate containing 565parts of aluminum nitrate and 0.8 part of free A1 0,. The filter cakeamounted to 336 parts and gave the characteristic X-ray pattern foralumina monohydrate. Recovery of the 141,0, was 59.8% of the totalaluminum (calculated as Algog) present in the originally chargedAI(NO,),.9H,O equivalent to 99.6% of the amount theoreticallyrecoverable from the portion of AI(NO,),.9- H,O denitrified. Recovery ofHNO, based on total aluminum nitrate charged was 60.0%.

EXAMPLE 4 A charge of 1,850 parts of AI(N0,),.9H,O was denitrified inthe manner described in Example 1, at 180C. and 50 psig until 50.3% ofthe theoretically possible N,O, had been removed and collected as 57.8%l-INO,

The partially denitrified liquor was diluted in the autoclave with 1,450parts of water containing 260 parts of washed product cake from Example1 as seed crystals. The autoclave was closed and the diluted seededliquor was heated to 200C. under an autogenous pressure of 250 psig anddigested under these conditions for 2 A hours. The autoclave was thencooled to zero gauge pressure and the product slurry removed andfiltered, and the cake washed with 340 parts of 1% Al(- N0,),.9H,Osolution. The solid product was in the form of easily filterablecrystals of considerably larger size than those obtained in Example 1 inwhich no seed crystals were added. The filtrate contained 6.05% aluminumnitrate (calculated as Al,0,) and 0.12% free HNO,. The combined nitratewas 22.2%. The wet filter cake contained 43.0% 141,0, equivalent to50.3% recovery based on the total Al(NO,) .9H,0 in the original chargeand of the amount theoretically recoverable from the portion ofA1(NO;,),.9H,O denitrified.

EXAMPLE 5 In the manner described in the foregoing examples, a charge of4,000 parts of Al( NO,),.9H,O was denitrified under 50 psig pressure and180C. temperature until 33.3% of the N 0,, had been removed as 45.1%HNO,. The denitrified liquor was diluted with 1,200 parts of water whichprovided a solution containing 14.6% A00, and 36.3% combined l-lN0,,based on the total weight of the solution. The diluted liquor was heatedto 160C. to give a maximum pressure of 50 psig for one hour. Theresulting thick slurry was removed from the depressurized autoclave andfiltered. The filtrate analyzed 12.5% AI,O=, and 44.5% l-lNO whichrepresented a change in molar ratio of Al,O,/HNO, from 1:4 in the diluteliquor to 1:577 in the filtrate, corresponding to nearly normal (1:6)Al(NO,),. The filter cake, after drying at C., contained only 50.4% A10, with a high HNO, content of 25.9% equivalent to a molar ratio AhozHNO of 110.83. This indicates that much of the Al,0 has precipitated asthe dibasic aluminum nitrate Al(Ol-[) N0 rather than as the desiredalumina monohydrate Al,0 .lH,0, and that the temperature of C. at whichthe heating of the dilute denitrified liquor was carried out, is too lowto effect complete hydrolytic disproportionation of the basic nitrate toalumina monohydrate.

EXAMPLE 6 A charge of 5400 parts of aluminum nitrate liquor prepared bypressure digestion of a kaolin clay at C. and containing 1420 parts ofaluminum nitrate (340 parts ALO, and 1260 parts HNO,), containing 6.3%aluminum calculated as Al,(), and .02% iron calculated as Fe,0, wasconcentrated by volatilization of 1,940 parts of water thus producing3460 parts of a liquor of 9.85% AI,O, content. This liquor was placedinto an autoclave and heated at C. and 50 psig pressure while ventingthe vaporous decomposition products. Heating and venting were continueduntil 50% of the combined HNO, theoretically obtainable from thealuminum nitrate in the charge had been removed and collected as 31%HNO,, leaving a basic aluminum nitrate in the autoclave.

The partially denitrified liquor produced above was diluted with 1,850parts of water containing 136 parts of alumina monohydrate as seed. Thediluted, seedcontaining liquor was heated for 2 hours at 200C. underpressure of 200 psig to convert the basic aluminum nitrate to aluminamonohydrate (Al O,.1-1,0) and aluminum nitrate. The resulting slurry,containing Al- ,0,.H,0 crystals of approximately 40 microns in diameterwas cooled and filtered and the filter cake washed. The resulting 6,760parts of filtrate and washings contained 710 parts of aluminum nitrateand 15 parts of free 141,0, dissolved therein. The 1,400 parts of wetfilter cake contained a total of 294 parts of alumina monohydrate ofwhich 158 parts was in excess over the seed crystals charged. The excessalumina monohydrate precipitated represents 46.4% of the alumina chargedin the digester liquor or 93.0% of the alumina denitrified. The recoveryof nitric values as nitric acid was 50.0% of the nitric acid equivalentin the digest liquor.

The total amount of N 0,, accounted for as aluminum nitrate in thefiltrate and as HNO; inthe recovered nitric acid and in themonohydratecrystals amounted to 99.95% of that originally present in thealuminum nitrate charged, of which 0.5% remain in the monohydratecrystals, and only 0.05% of the total N 0,, lost as NO and N 1 A portionof the recovered crystals of Al o H O were calcined by heating at 900C.for one hour, during which time nitric acid vapors were taken off andcol lected and amounted to only 0.5% based on that originally present inthe aluminum nitrate charged. No dust was detectable in these vapors.The resulting calcined A1 0 had an average particle size considerably inexcess of the micron size of the original crystals and ranged from about50 microns to about 100 microns.

When the 52.6% denitrified material was recovered as described inExamples 3 or 4 of U.S. Pat. No. 3,366,446, by flashing to release thepressure, followed by drying at 130C. for 1 hour and finally bycalcining the solidified melt at 420C. (800F.) and passing the resultingvapors successively through a caustic absorber for N0 and a permanganateabsorber for NO, the condensate which was collected contained 0.9% ofthe total alumina carried over into the condensate. This nitric acidcondensate (containing HNO from the denitrification flashing andcalcining steps) contained only 81% of the nitric acid in the chargedaluminum nitrate. The NO, and NO absorbers contained 12.1% of the totaloriginal HNO, as NO, and 6.9% as NO. A total of 19% of the original HN0converted to lower oxides of nitrogen, as compared to only 0.55% in theprocess of our invention as described above.

EXAMPLE 7 1n the manner described in Example 1, a charge of 1000 partsof Al(NO,) .9H,O) was denitrified under 50 psig pressure and 180C.temperature until 24.5% of the N 0 had been removed as 48.3% HNO Thepartially denitrifled liquor was diluted with 3,050 parts of water whichprovided a solution containing a total aluminum concentration of 3.58%calculated as Al,0,, and 10.0% combined HNO based on the total weight ofthe solution. The diluted liquor was heated to 200C. thus producing amaximum pressure of 200 psig for 4 hours. The resulting slurry wasremoved from the depressurized autoclave and filtered. The filtrateanalyzed 3.18% A1 0: and 10.4% HNO, which represented a change in molarratio of A1,O /HNO from 1:4.53 in the dilute liquor to 1:5.29 in thefiltrate. The resulting 28 parts of dried filter cake contained 19.3parts A1 0 of which 18.5 parts was present as the monohydrate. (The restwas combined with occluded nitrate). The 18.5 parts A1 0 represents13.6% of the 136 parts A1 0 charged to the autoclave or 55.5% of the33.3 parts of denitrified Also; in the diluted liquor. Recovery of A111) was thus 13.6% of the total aluminum present in the originallycharged Al(NO,) .9H,O, and 55.5% of the amount theoretically recoverablefrom the portion of Al(NO,) .9H,O denitritied.

EXAMPLE 8 A charge of 2000 parts of Al(NO,),.9l-l,0 was denitrified byheating at temperatures between 130 and 140C. under atmospheric pressureuntil 50% of the N 0 in the charge had been removed and collected as 50%HNO The resulting gummy melt was poured into a tray, forming a thin filmwhich was cooled to room temperature (Ca. 25C). The resulting congealedmelt was broken up into small pieces which were transferred to a vesseland carefully heated over a range progressing from about 100C. to about200C. during which the melt slowly powdered and an additional 25% of N 0was recovered and collected as HNO The partially denitrified solid thusobtained was charged to an autoclave with 2,100 parts of water, thusproviding a total aluminum concentration of 10% calculated as A1 0, anda combined HNO concentration of 10%. The autoclave was closed and thediluted charge was heated to 200C. at a pressure of 220 psig for 3hours. The resulting slurry was removed from the autoclave and filtered,yielding 2,460 parts of filtrate containing 252 parts of aluminumnitrate and 4 parts of free A1 0 The filter cake amounted to 214 partsof which 40 parts were combined with occluded nitrate. The 210 parts ofA1 0 were present as the monohydrate and represented a recovery of 76.5%of the total aluminum in the aluminum nitrate charged and of thealuminum in the partially denitrified material.

EXAMPLE 9 A charge of aluminum nitrate nonahydrate was denitrified at C.and 50 psig pressure until 50.2% of the possible N 0 had been removed.Then 50 parts of this partially denitrified material, containing 12.7parts aluminum calculated as A1 0 23.6 parts of HNO and 13.65 parts ofwater was further denitrified by heating at 420C. in an agitated flask,while collecting the emitted vapors and analyzing them periodically forparts HNO, (composed of nitric acid values released as N 0 and condensedwith water to form HNO for N0 and NO emitted, the NO, being collected bycaustic absorption, the NO in caustic and permanganate.

Results are shown in Table 11 below.

TABLE I1 Variations in Composition of Nitric Acid Decomposition ProductsProduced at Different degrees of Denitrification of A1(NO,)-Denitrification Parts in Condensate It is apparent from Table ll thatdenitrification beyond about 75% results in increasing decomposition ofthe nitric acid values to lower oxides of nitrogen NO, and NO at theexpense of N 0,, (recorded as HNO While the above describes thepreferred embodiments of our invention, it will be understood thatdepartures can be made therefrom within the scope of the specificationand claims.

We claim:

I. In a process for recovering alumina monohydrate and aluminum nitratefrom basic aluminum nitrates prepared by melting aluminum nitratenonahydrate, partially denitrifying said molten aluminum nitratenonahydrate by heating it to a temperature above its decompositiontemperature but not above about 235C. under superatmospheric pressure ofat least about 25 psig and sufficient to maintain said decompositiontemperature, while removing gaseous nitric acid decomposition productsuntil between about 45% and about 55% of the l-lNO of the aluminumnitrate 12 about 235C. under autogenous pressure for a period sufficientto convert the basic aluminum nitrate to solid alumina monohydrate anddissolved aluminum nitrate wherein oxides of nitrogen released therefromare substantially N 0 (3) recovering the solid alumina monohydrate byseparating the same from the solution, (4) recycling the separatedsolution to a succeeding aluminum nitrate dentrification step, whereby asteady state is established wherein recovery of both alumina and nitricacid is high, and (5) recovering approximately 99.5% of the HNO;originally present in said aluminum nitrate nonahydrate.

1. IN A PROCESS FOR RECOVERING ALUMINA MONOHYDRATE AND ALUMINUM NITRATEFROM BASE ALUMINUM NITRATES PREPAARED BY MELTING ALUMINUM NITRATENONAHYDRATE, PARTIALLY DENITRIFYING SAID MOLTEN ALUMINUM NITRATENONAHYDRATE BY HEATING IT TO A TEMPERATURE ABOVE ITS DECOMPOSITIONTEMPERATURE BUT NOT ABOVE ABOUT 235*C. UNDER SUPERATMOSPHERIC PRESSUREOF AT LEAST ABOUT 25 PSIG AND SUFFICIENT TO MAINTAIN SAID DECOMPOSITIONTEMPERATURE, WHILE REMOVING GASEOUS NITRIC ACID DECOMPOSITION PRODUCTSUNTIL BEETWEEN ABOUT 45% AND ABOUT 55% OF THE HNO3 OF THE ALUMINUMNITRATE NONAHYDRATE HAS BEEN REMOVED, THE STEP WHICH COMPRISE
 1. ADDINGTO A BASIC ALUMINUM NITATE HAVING THE EMPIRICAL FORMULA